The invention is directed to a method for determining the relative humidity of gaseous materials, perferably for determining the relative humidity of air in rooms, such as greenhouses, whereby, using psychrometric moisture measurement, the temperature of the stream of gas, the relative humidity of which is to be measured, is determined by means of a dry temperature sensor, which cools a gas stream acting upon a moistenable wet temperature sensor adiabatically up to the cooling limit (wet-bulb point) with determination of the wet bulb point temperature, and the relative humidity is ascertained using the measured temperature of the gas and the wet-bulb temperature. It is furthermore an object of the invention to provide an apparatus for carrying out a method for determining the relative humidity of gaseous materials in accordance with the introductory portion of claim 4.
The measurement and determination of the relative humidity of air is increasingly required for many different applications, for example, for controlling the heating and air-conditioning facilities in warehouses for the optimized warehousing of perishable goods, in medical facilities, such as operating rooms, in production buildings, for processing and handling hygroscopic materials, such as paper, textiles, tobacco or grain, but particularly in buildings, which are used for raising agricultural and also horticultural products. However, conditions, which create appreciable problems, exist in horticultural operations such as those conducted in greenhouses. For example, problems arise during the evening and night hours due to the radiation of heat or a lowering of the temperature, which result in an increase in the humidity of the air in the greenhouse. This increase in humidity leads to undesirable plant reactions, such as the bursting of buds, growth disorders, etc., when certain plant-specific limits are exceeded. The dropping of the temperature below the dew point also can also have particularly disadvantageous effects, since it furthers the massive occurrence of diseases, destructive insects, parasites and physiological disorders such as bud discoloration and can make it necessary, in a particular case, to use fungicides, which are expensive and not without risk. This makes it necessary to assign special importance to the climatic conditions.
Conventionally, the temperature of a horticultural production plant is controlled by alternately heating and venting. This is done party manually, but to an increasing degree by automatic means. However, this presupposes that, for controlling the temperature, methods and equipment for measuring the humidity are available, which are able to measure or determine the existing air humidity accurately and, moreover, also over longer periods of time and also in the high humidity range from 85 to 100% relative humidity.
Different methods are known, with the help of which the relative humidity can be determined. Equipment of the usual construction for measuring the humidity of air at the present time works according to the so-called evaporation method using the psychrometric measurement of humidity, which is based on the constant interchange of water vapor between water or ice and the surrounding atmosphere. In the case of the known equipment, which works according to this principle, the so-called psychrometers, a wet temperature sensor is provided with a textile sock, which is moistened from a water reservoir. The textile sock of the wet temperature sensor is disposed in the measuring chamber of the psychrometer and is acted upon by the air stream. In so doing, the air flowing past is cooled until it is saturated, that is, it is cooled up to the so-called wet-bulb point. This wet-bulb temperature is determined with the wet temperature sensor. The cooling up to the wet-bulb point should take place adiabatically, so that the heat of evaporation must be supplied exclusively by the air passing by. Accordingly, appropriate actions must be taken to ensure that heat is not supplied by the equipment itself, by radiation, from the water reservoir, etc.
The difference between the air temperature, which is determined with the dry temperature sensor, and the wet-bulb temperature is referred to as the psychrometric difference. The relative humidity can then be determined from an enthalpy/water content diagram (h-x diagram) with the help of the surrounding temperature and the wet-bulb point temperature. If modern data processing equipment is used, this can also be done by computer.
It is a disadvantage of psychrometers of the known construction that they cannot be used efficiently for measurements over long periods of time. The stocking must be wetted again at intervals of 10 to 15 minutes. Moreover, distilled water is required for the moistening, because the cotton stocking loses its absorptivity due to lime deposits and because salt deposits lower the vapor pressure. Before the measurement, the instruments must have been in the atmosphere that is to be measured long enough, so that their temperature matches that of the surroundings. If such instruments are to be used for continuous methods of measuring, as would be required, for example, by a modern heating and cooling control system in horticultural installations, limits are set. This is very disadvantageous, particularly for measurements in the high humidity range. In horticultural practice, where it is necessary to measure in the high humidity range, the effective control of the heating and cooling equipment with respect to controlling the temperature accurately therefore frequently fails to measure the air humidity accurately as a controlled variable over prolonged periods of time. This can be attributed to the unreliability or inaccuracy of such known instruments or their components.